Where are the terms and conditions, and where can you get them from, if they are not available at that time of night then you could (reasonably) argue that it is an unfair contract (and it is a contract you enter into when you park on private land) and therefore unenforceable.
If you decide to argue this I can give you a list of case-law to refer them to and see how they respond (usually with a "don't do it again" letter, last thing they want is for a case to be lost and everyone they have ever fined to appeal based upon it.
It is also of interest to note that as McDonalds is open at that time and the other shops close at 8pm then by changing the time limit at that time when a store is still open you are, as a McDonalds customer, being discrimiated against
Yes I know, I don't like parking companies though.
Oh, almost forgot, as it's an unmanned carpark and they do it all by cameras there are a LOT of rules about signs, data protection act, storage, who has access to the files, how long they are kept for etc etc that they have to follow, along with calibration and adjustment of the time-stamp on all the pictures, who does it, how, are they certified, etc etc You are entitled to ask these things before parting (if you have to) with any money.